Simulation of Ungrounded Shipboard Power Systems in PSpice

Simulation of Ungrounded Shipboard Power Systems in PSpice Haibo Zhang IEEE Student Member Karen L.Butler IEEE Member Power System Automation Lab Electrical Engineering Department Texas A&M University College Station, TX 77843-3128 N.D.R.Sarma IEEE Member Abstract Navy shipboard power systems(spss) have different characteristics when compared to utility power systems. To conduct system studies on SPSs, an effective simulation tool is required. PSpice is a very robust analog/digital circuit simulator, but it is rarely used in power system studies. This paper presents a PSpice methodology for modeling SPSs. Transient simulation results are also presented. I. Introduction Ungrounded delta type system configuration is widely used in U.S. Navy shipboard electric power distribution systems with the objective of providing continuity of power supply. To conduct system analysis, such as fault analysis, system reconfiguration/restoration, etc. for Shipboard Power Systems (SPSs), an effective simulation tool is needed. In the literature, there is no information discussing the appropriate simulation tool for simulating SPSs. PSpice[1,2,3] is a popular simulation tools used for analog/digital circuits and power electronics. In this paper, modeling and simulation of SPSs using PSpice are discussed. In section II, SPSs are briefly described. Section III explains modeling of various components of SPS using PSpice. The details of the test system and the case studies conducted using PSpice are presented in section IV. Concluding remarks are given in section V. II. Shipboard power systems (SPSs) Navy ships use three phase power generated and distributed in an ungrounded delta configuration, which is used to ensure continuous operation of the power system in the presence of a single phase to hull fault [4,5]. The voltages are generated at levels of 450 volts a.c. at 60 hertz. The most popular topology used in SPSs is a ring configuration of the generators which allows any generator to provide power to any load. This feature is of great importance in order to ensure supply of power to vital loads if an operating generating unit fails. Generators are connected to switchboards called as generator switchboards which are composed of one or more switchgear units and breakers for various devices like generators, loads, and load centers. These generator switchboards are located close to their associated generators. Bus tie circuits that interconnect the generator switchboards allow for the transfer of power from one switchboard to another. The other components of SPSs are load centers, power distribution panels, bus transfer units, transformers, and cable used for delivering power to the loads. Load center distribution, which is a modification of radial distribution, is used below the generator switchboard level. One or more load centers are connected to each generator switchboard to supply power to load concentrations in various areas of the ship. The load centers supply power to power panels or individual loads, either directly or via automatic bus transfers (ABTs) or manual bus transfers (MBTs). For vital loads, two separate sources of power (normal and alternate) are provided via ABTs and MBTs. Loads are fed directly from generator switchboards, from load centers, or from power distribution panels. The loads require power at 440, 115, and 4,160 volts at 60 hertz, and 440 and 115 volts at 400 hertz. Transformers are used to convert supply voltage from one level to another. The interfaces used between 60 hertz and 400 hertz systems are either motor-generator sets or solid-state frequency converters (SSFCs). Although SPSs appear simpler than their commercial terrestrial counterparts, they have unique characteristics that demand special attention during designing and

performing other kinds of investigations, such as fault studies, reconfiguration, etc. To conduct system studies, it is required to model various components of the SPSs and conduct simulation studies. In the present study, PSpice has been used to simulate SPS. Initially various components of SPS are modeled using the basic elements available in PSpice. Later these modules are used to build a simple test system to conduct simulation studies. In the current studies, protective devices are not modeled. III. Modeling of SPSs using PSpice PSpice, a computer software developed by Microsim Corporation, is considered to be one of the most powerful and popular analog circuit simulators[1]. PSpice uses linear, discrete, passive and active electrical elements for network simulation. Here PSpice is used to simulate a simple test system which includes components common to SPSs. Following sections will show how these components are modeled in PSpice. Since PSpice does not allow the floating condition, very large resistors (10 7 ohms) are used to avoid the floating condition whenever required. A. Generator model In PSpice, the voltage sources can be connected in delta which are used directly to build the generator sets. The generator set is modeled in PSpice as shown in Fig. 1 with the shunt resistors (to avoid floating condition) and source impedance. The parameters of the generator model include: voltage magnitude, frequency, source impedance of each phase, shunt impedance of each phase. B. Transformer model Fig.1 Generator model Three-phase delta-connected transformer banks are modeled as three single-phase transformers connected in delta. PSpice provides modeling of a single-phase transformer with only primary and secondary inductance. Hence the primary and secondary resistance are added to the basic transformer model provided in PSpice. The transformer model is shown in Fig.2. The parameters for transformer model include: primary/secondary inductance/resistance, coupling coefficient. C. Cable model Fig.2 Transformer model Using PSpice, cables of SPSs are modeled as a lumped π-model whose component values are calculated using the distributed resistance r, self-inductance l s, selfcapacitance c s, mutual inductance l m and mutual capacitance c m. The values of these parameters depend on the type of the cable. The parameters of the cable model include: distributed r, l s,l m, c s, c m, and the cable length. The cable model is shown in Fig 3. D. Three phase switches Fig.3 Cable model PSpice provides two types of single-phase timecontrolled switches, one initially closed and another initially open, which are used to model circuit breakers and switches. In SPSs modeling, two types of three phase switches are modeled. One is normally closed and another is normally open. The parameters of the three phase switches include the open / close time for each phase, transient time, the resistance across the switch when it is open or closed. This switch model is also used to simulate a fault. E. ABT/MBT model L ab L bc L ac Automatic Bus Transfer switch (ABT) or Manual Bus Transfer switch (MBT) provide the normal and alternate path to vital loads. The bus transfer switch model is shown in Fig.4. It is composed of one 3-phase normally closed switch 1 (connecting node 1 and 2 as normal path)

and one 3-phase normally open switch 2 (connecting node 1 and 3 as alternate path). When the path through node 2 is not available to node 1, switch 1 and 2 will exchange their status at the same time. The parameters of the ABT/MBT model include the switch on/off time, transient time, the resistance values in open and closed states. normal path Bus Transfer Switch (ABT or MBT) Load alternate path 2 3 (a) ABT/MBT function (b) ABT/MBT model Fig. 4. ABT/MBT model F. Switchboard, load center and power panel Generator switchboards are composed of one or more switchgear units and breakers for various devices like generators, loads, and load centers. Bus tie switches that interconnect the generator switchboards allow for the transfer of power from one switchboard to another. The generator switchboard is modeled as shown in Fig. 5. The parameters of the switchboard include the parameters of each switch. For load center and power panel, the model is similar to that of generator switchboard but without the bus tie switches. Bus-tie breaker G. load model 1 Load To load centers/power panels/loads From generator Fig 5. Switchboard model Bus-tie breaker Accordingly, the configuration of induction motor model is shown in Fig.6. va vb vc 3 phase ->dq axis vqs vds stator flux ωr ia ib ic iqs ids torque T stator vs. rotor flux iqs ids dq axis-> 3 phase Tl iqr idr Fig 6. Configuration of Induction Motor load torque rotor flux The parameters needed for the induction motor model are: stator resistance, stator leakage inductance, magnetizing inductance, number of pole pairs, synchronous speed, bigger/smaller cage rotor resistance, bigger/smaller cage rotor leakage inductance. IV Test system and case studies A. Test system The radial test system shown in Fig. 7 is simulated using components models developed in the previous section. The component specifications are as follows: Generator G: 12 KVA, 450 V line-to-line, 60 Hz Transformer T: 5 KVA, 450/115V line-to-line, - connected Load L1: 2 kw, PF = 1.0, 450 V line-to-line, balanced, -connected Load L2: 5 KVA, PF = 0.8, 115V line-to-line, balanced, -connected Induction motor M1: 5.63 HP, 450 V line-to-line, synchronous speed = 3600 rpm Cable C1: 3-φ, 350 MCM, length = 12.2 meters Cable C2: 3-φ, 6 AWG, length = 20 meters Cable C3: 3-φ, 6 AWG, length = 12.2 meters Cable C4: 3-φ, 6 AWG, length = 12 meters. Loads in SPSs are either rotating or non-rotating. The non-rotating loads are constant, passive auxiliary loads such as lighting, heaters, etc. These kinds of loads are modeled as combinations of resistor and inductor, depending on their respective power factors, voltage and power ratings. In SPSs, induction motor is the most common rotating type load that is used for water and fire pumps, compressors, and other applications that may consist of either continuous or intermittent operation. Based on the circuit equations, a double cage induction motor model [6] is implemented using amplifier, difference junction and integrator, etc. 450 V line-to-line, 60 Hz 12 KVA delta-connected generator G generator switchboard GEN cable C1 FAULT POINT F switchboard SB cable C3 cable C4 M induction motor M1: cable C2 5.63 HP, PF = 0.883 450/115 V 5 KVA delta-delta transformer T load L2: load L1: 5 kva, PF = 0.8 2 kw, PF = 1.0 Fig.7 The test system

Three different scenarios are developed to study the given test system under fault conditions. Case I: Normal system operation (no fault) Case II: Single-Line-to-Hull(SLH) fault between phase A and hull in the middle of cable C1 Case III: Line-to-Line (2L) fault between phase B and C in the middle of cable C1 B. Simulation results In this section, the results of the simulations will be discussed. Each case is illustrated with waveforms. PSpice adjusts the time step for simulation automatically, but one can define the maximum time step called as the step ceiling. For all the simulations in current studies, the step ceiling is chosen as 100µs. changes its direction of rotation and operates in braking mode, which leads to overheating of the motor. Normally the stator is disconnected before the braking mode. Since we haven t modeled the protective devices here, it s not shown in the simulation result. From Fig. 16, it can be seen that the torque begins to oscillate at fault time. The phase a current of the motor is as shown in Fig. 17 which indicates that it increases after the fault. From the results in case III, it can be seen that L-L fault is a serious fault for SPSs, it influences the system voltages, currents and also the load characteristics. Case I: The waveform of induction motor phase a current is shown in Fig. 8. The speed and torque waveforms for the motor are shown in figs. 9 and 10 respectively. The steady state values are obtained as expected (torque = 11.17 N-m, I a = 8.17 A ) Case II: In this case the single line to hull fault is staged at 2.060s in the middle of cable C1 at point F. The lineto-line and line-to-hull (V ah ) voltage waveforms for this case are as shown in Fig. 11. It can be seen that there is a little spike in the waveforms of line-to-line voltages V bc and V ca at the fault time. The steady state values of lineto-line voltages are the same as in case I. This illustrates that an ungrounded delta configuration ensures normal voltage supply even in the presence of a single line to hull fault. This behavior also explains why navy ship designers adopted the delta configuration. The waveform of induction motor phase a current is the same as in case I. There is no change for the induction motor phase a current even the single line to hull fault occurred in phase a. Fig.12 shows the waveforms of the induction motor torque. At the fault time, there is a little transient. Fig.13 gives the waveforms of transformer primary and secondary line to line voltages V ab and V a b. The line to line voltages remain same with an exception that at the fault time, there is a small spike on both waveforms. Case III. In this case, the line to line fault is staged at 2.060s between phase b and c in the middle of cable C1. Fig.14 gives the waveforms of line-to-line voltages V ab, V bc, V ca at the fault point F. Fig.15 gives the waveform of induction motor speed. When there is a L-L fault, the speed decreases and becomes zero(standstill state) at about 1.6 seconds after the fault. From this moment, the induction motor Fig.8 Induction motor phase a current (casei) V ab V bc V ca Fig.9 Induction motor speed (case I) Fig.10 Induction motor Torque(case I ) V ah Fig. 11 Line-to-line voltages and line to hull voltage V ah at point F ( Case II)

Fig. 17 Induction motor phase a current (caseiii) V. Conclusion V ab V bc V ca Fig. 12 Induction motor torque(case II) V ab V a b Fig. 13 Transformer primary/secondary voltages V ab and V a b (case II) SPSs components are modeled in PSpice. A test system containing typical components of Shipboard Power Systems (SPSs) has been simulated using PSpice. Different case studies have been discussed. PSpice has an excellent graphical interface for building complex SPSs and flexibility in establishing monitoring points and processing output data. However, PSpice must be given basic circuit parameters for each component model, which can be obtained from other power system simulation tools or can be directly calculated. In conclusion, PSpice is a promising tool for simulating SPSs. VI. Acknowledgments The authors acknowledge the Office of Naval Research for the support of this project through grant N00014-96-1-0523. VII. References Fig. 14 Line-to-line voltages at point F (case III) Fig.15 Induction motor speed (case III) Fig. 16 Induction motor torque (case III) [1] PSpice A/D Simulator, MicroSim Corporation, ver 8.0, July 1997. [2] K.J.Tseng, C.F. Foo and P.R.Palmer, Implementing Power Doide Models in SPICE and Saber, 25th IEEE Annual Power Electronics Specialists Conference Proceedings, PESC 94, June 20-24,1994, pp 59-63. [3] Zhixin Yan,M.J. Deen, New RTD large-signal DC model suitabl for PSPICE, IEEE-Transactions on Computer Aided Design of Integrated Circuits and Systems. v. 14 Feb. '95 p. 167-72 [4] Naval Sea Systems Command, Engineering Directorate, Electrical Engineering Group, NAVSEA Design Practice and Criteria Manual for Electrical Systems for Surface Ships, Chapter 300. [5] K.L.Butler, N.D.R.Sarma, C.A. Whitcomb, H.J.DoCarmo and H.Zhang, Shipboard Power Systems Deploy Automated Protections, IEEE Computer Applications in Power, Volume 11, No.2, April, 1998, pp 31-36 [6] Junichi Arai, Yasuhiro Noro, An Induction Machine Model for an Analog Power System Simulator, IEEE Transactions on Power Systems, Vol.8, No.4, November 1994, pp1478-1482